In the case of a general air conditioning system in the related art, which is used in a battery system, such as a battery container for an energy storage system (ESS) including a plurality of battery racks, a temperature deviation between battery modules is inevitably generated according to positions of the battery rack and the battery module.
To describe in detail, the temperature deviation between the battery modules includes a temperature deviation portion according to a position by an air conditioning method, in addition to a self temperature increase portion through charging/discharging. A general air conditioning method in the related art in a battery system is a method of cooling a surface of a battery module while moving cold cooling air from a floor (or a ceiling) to the ceiling (or the floor). In the general air conditioning method in the related art, a temperature of the battery module close to a place where cold air is discharged is lower than that of the battery module positioned at an end of an opposite side of the place where cold air is discharged, that is, at a place where the cooling air after decreasing an overall temperature of the battery system enters. The reason is that the temperature of the cooling air itself is increased while the cooling air passes through a surface of each of the battery modules.
The temperature deviation between the battery modules according to a position in the general air conditioning system in the related art generally exerts a bad effect to two parts.
First, a depth of discharge (DOD) is decreased due to a temperature deviation diagnosis function of a battery system. The battery system adjusts a temperature deviation of a battery module not to exceed a predetermined level during a charging/discharging process through an autonomous temperature deviation diagnosis function. Accordingly, when the battery system fails to rapidly adjust the temperature deviation, so that the temperature deviation between the battery modules of the battery system exceeds the predetermined level, the temperature deviation diagnosis function of the battery system is operated, so that the battery system cannot sufficiently charging/discharging and may stop the charging/discharging. This results in damage to the DOD of the battery system.
Second, a state of health (SOH) (%) deviation is generated between the battery modules included in the battery system. A lithium ion cell, which is a main component of a lithium ion battery module mainly used in a power storage field, has a characteristic in that an SOH is different according to an operation temperature. For example, the more charging/discharging is repeated over a long time at a high temperature, rather than room temperature, the faster a degradation speed is. Accordingly, when a temperature deviation between the battery modules is continued over a long time due to a limit in solving a temperature deviation in the general air conditioning system in the related art in the battery system, a difference in an SOH between the battery modules may be increased. Further, when an SOH deviation between the racks/modules is increased, a battery capacity (state of charge (SOC)) or a voltage of the rack/module (more accurately, a specific cell) having a small SOH first reaches a charging/discharging limit value (0% or 100% of the SOC) than other racks/modules during the charging/discharging. Accordingly, even in this case, in the battery system, the SOC diagnosis function or a voltage diagnosis function is operated, so that the charging/discharging of the battery system may be stopped, and thus the DOD may be damaged.